This invention relates to composite powders of ceramic/metal of the type comprising a matrix of alumina and at least one transition metal, in particular iron, chromium, molybdenum, cobalt, nickel or niobium, which is present in the form of a dispersion of fine particles on the interior of each grain of the matrix. The invention also relates to powders called "nano-composites" in which the metal particles are on the scale of a nanometer. The invention also relates to cermets obtained by sintering said powders and to processes for production of these powders and cermets.
Composite alumina/metal powders are used for producing catalysts, or for fabrication by sintering cermets. The latter have numerous applications in various sectors of industry by reason of their thermo-mechanical and dielectric properties (by the term "cermet" is customarily meant a composite material mass of ceramic/metal).
Composite alumina/metal powders presently known are essentially of two types:
microcrystalline powders of high porosity for production of catalyst supports (J. J. Chen, E. Ruckenstein, Journal of Catalysts 69, (1981), 254-273; A. Uneo, H. Suzuki, Y. Kotera, Journal of the Chemical Society, Faraday Transaction 79, (1983), 127-136), PA1 amorphous gels (L. Ganapathi et al, Journ. Solid State Ch., vol. 66, 1987, pages 376-378; J. T. Klomp et al, Ceramurgia int., vol. 4, 1978, pages 59-65; D. Chakravorty, Sadhana, vol. 13, 1988, pages 13-18 . . . ). PA1 each grain comprises a compact matrix, of a specific surface area less than 5 m.sup.2 /g, PA1 said matrix is comprised of .alpha. alumina (corundum) of hexagonal structure, PA1 the transition metal or metals are dispersed in each grain at the center of the alumina matrix in the form of small crystals of sizes less than 50 nanometers (designated below as "nanocrystals"), PA1 the weight ratio of metal/alumina is less than 30%. PA1 a minor addition to the powder of an organic binder having a decomposition temperature of between 150.degree. C. and 300.degree. C., decomposition being accompanied by a release of CO, PA1 compacting of the powder/binder mixture, PA1 heating of the compacted mixture, under a neutral atmosphere or reduced pressure, at a temperature of between 1350.degree. C. and 1550.degree. C. PA1 a resistance to bending comprising between 500 and 1000 megapascals, PA1 a resilience comprising between 5 and 10 megapascals per .sqroot.m. PA1 the use of a mixed precursor, PA1 the precipitation conditions thereof, PA1 the decomposition conditions of the precursor, PA1 the heat treatment conditions of the decomposition residues.
These powders are comprised of matrices of .gamma. or amorphous alumina and a metallic dispersion. Their main drawback is in being thermally unstable such that they do not permit production by sintering of cermets having good mechanical properties. During sintering, the metal particles have, in effect, a tendency to coalesce and to migrate toward their grain junctions leading to a heterogeneous dispersion in which the metal phase is found in the form of particles of large size juxtaposed with grains of ceramic, with appearance of porosities between these phases. This microstructure leads to poor properties in a mechanical sense, and a thermal instability arising from differences of the coefficients of expansion of the phases (which causes a rupturing of the structure during heat treatment).
Certain prior art documents have suggested the possibility of mixing .alpha. alumina with a metal binder (patent DD 137,313; U.S. Pat. No. 4,397,963, "Proceeding of the 21st automotive technology coordination meeting," March, 1984, Society of Automotive Engineers Inc., Warrendale, Pa., USA; American Ceramic Society Bulletin, vol. 61, no. 9, September, 1982, Columbus U.S. pages 974-981, C. S. Morgan, et al: Thermal-Shock Resistant Alumina-metal Cermet Insulators"). However, in the powders thus obtained, the metal is arranged on the periphery of the grains: these powders have a microstructure which is fundamentally different from that provided by the present invention, since the metal is not inserted on the interior of each grain of the matrix. The sintering of these powders leads to microstructures of cermets similar to those indicated above with inherent drawbacks.
The present invention proposes providing new alumina/metal composite powders in which the metal is present in the form of a dispersion on the interior of each grain of the matrix, and their process of production. The invention seeks to overcome the drawbacks of known powders and permits the production of cermets benefitting from greatly improved mechanical properties and a good thermo-mechanical relationship.
In particular, the invention seeks to permit the production of alumina/metal cermets which are apt to sustain thermal shocks.
Another object of the invention is to permit production of powders from several metals, in which the metal dispersion is in the form of an alloy in order to benefit from the properties of the latter.